1. Field of the Invention
The present invention relates to an X-ray tube of the rotary anode type and, more particularly, an improvement of bearing components suitable for use with the X-ray tube of the rotary anode type.
2. Description of the Related Art
As well known, in an X-ray tube of the rotary anode type, a disk-like anode target is fixed to a rotary structure which is rotatably supported by a stationary structure and bearings are formed between the stationary and rotary structures. An electron beam is bombarded on the anode target so that X-rays are radiated from the anode target, while exciting electromagnetic coils located outside a vacuum envelope to rotate the rotary structure at high speed. Ball bearings have been used for a long time but it is now expected that bearings of the hydro-dynamic pressure type will become used. In the case of these dynamic pressure type bearings, spiral grooves are formed on the bearing face and liquid metal such as gallium (Ga) and alloy of gallium, indium and tin (Ga-In-Sn) are used as lubricants. Examples in which the dynamic pressure type bearings are used are disclosed in Japanese Patent Publication 60-21463, and Japanese Patent Disclosures 60-97536, 60-117531, 62-287555, 2-227947 and 2-227948, for example.
In the case of the X-ray tube of the rotary anode type in which the above-mentioned liquid metal lubricant is used together with the hydro-dynamic pressure type slide bearings, spiral grooves of the herringbone pattern are used. When the rotary structure is being rotated, the lubricant is collected from the peripheral portion of the herringbone pattern to the center thereof to generate dynamic pressure. Each bearing face is kept apart from its corresponding face of the rotary or stationary structures with an interval of 20 micrometers. That separation is for example, interposed between them while the rotary structure is being rotated. While the rotary structure is stopped, however, a part of the bearing face is contacted with the corresponding face of the fixed or rotary structure because of the weight of the rotary structure. If the lubricant is present, as a thin film, between these contacted faces, the bearing face will not be damaged even when the contacted faces are rubbed against each other at the time of an operation start. When the rotary structure is stopped, however, the lubricant is not sometimes present on that area of the bearing face where the lubricant becomes thinner or less when the X-ray tube is under operation. If no lubricant is present on the bearing face, the contacted faces are rubbed against each other to damage or bake the bearing face when the rotary structure again starts its rotation.